1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a dual-frequency bistable liquid crystal display and the liquid crystal mixture thereof.
2. Description of the Prior Art
The special electro-optic characteristic of the liquid crystal is that the liquid crystal is able to display images after being electrified. However, for some electronic equipments, such as the E-reader, smart phone, and electronic tag, and etc., there is strict requirements on the display to be used, for example, the power consumption should be very low and service life should be longer than, say, five years. Therefore, a bistable liquid crystal display made of nematic liquid crystal or cholesteric liquid crystal with appropriate device parameter and relative material parameters is a good solution. Then voltage is used to induce two types of alignments of the liquid crystal, so as to display images with a certain contrast level. Once the image is formed, the voltage doesn't have to be maintained, which satisfies the requirements for low power consumption and long life.
The nematic liquid crystal display generally includes ZBD (Zenithal Bitable Display) and BiNem display (Nemoptic), both of which use specially designed surface alignment layer to achieve the bistability. The cholesteric liquid crystal is formed by mixing of nematic liquid crystal and chiral compound, and then the mixture of the nematic liquid crystal and chiral compound helically deforms to form the cholesteric liquid crystal. Cholesteric liquid crystal mainly includes SSCT (surface stabilized cholesteric texture) which applies positive cholesteric liquid crystal having positive dielectric anisotropy. The helical pitch of cholesteric liquid crystal is within visible light range. When the cholesteric liquid crystal is applied with a high enough voltage, the helical structure of the positive cholesteric liquid crystal will deform into nematic alignment structure which is unstable and perpendicular to the substrate layer. The unstable structure will be destabilized into planar texture or focal-conic texture when applied with a voltage, which induces two types of stable alignments of the cholesteric liquid crystal, forming the bistable liquid crystal display.
Merck developed a dual-frequency nematic liquid crystal (MLC-2048), as compared to the nematic liquid crystal whose alignment is controlled by voltage, the alignment of dual-frequency nematic liquid crystal is controlled by the frequency as well. Furthermore, the dielectric anisotropy (Δε) of the dual-frequency nematic liquid crystal is positive (Δε>0), when the frequency is relatively low, Δε>0 when the frequency is relatively high. As compared to the ordinary nematic liquid crystal, dual-frequency nematic liquid crystal possesses the characteristics of both positive and negative nematic liquid crystal, and therefore has a faster response time and is more suitable for producing high performance optical and display equipments.
However, when switching between two stable states of alignments (perpendicular ⇄parallel, or one parallel to another parallel), the ordinary nematic liquid crystal nematic liquid crystal will experience a metastable stage, which still has the problem of high power consumption. Besides, the SSCT, after removal of voltage, will has the problem of longer response time when switching between two stable states due to complicated electric fluid dynamics.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.